5.64. Decrement Memory by a Positive Integer

Addr
Mode  Mnemonic          Format/Opcode
                           8      4       4           16
D     DECM  N,ADDR      ---------------------------------------
DX    DECM  N,ADDR,RX   |  B3  |  N-1  |  RX  |  |    ADDR    |
                        ---------------------------------------

Description. The contents of the memory location specified by the Derived Address, DA, are decremented by N, where N is an integer, 1 <= N <= 16. This is equivalent of a "subtract-from-memory instruction". The condition status, CS, is set based on the results of the subtraction and carry. A fixed point overflow occurs if the operand in memory is negative and the result is positive. The memory location specified is updated to contain the result of the subtraction process even if a fixed point overflow occurs.

Register Transfer Description.

[DA]² <-- [DA]¹ - N, where 1 <= N <= 16;
PI₄ <-- 1, if [DA₀] ¹ < 0 < [DA₀] ²;
(CS) <-- 0010  if  carry = 0 and [DA] = 0;
(CS) <-- 0001  if  carry = 0 and [DA] < 0;
(CS) <-- 0100  if  carry = 0 and [DA] > 0;
(CS) <-- 1010  if  carry = 1 and [DA] = 0;
(CS) <-- 1001  if  carry = 1 and [DA] < 0;
(CS) <-- 1100  if  carry = 1 and [DA] > 0;

Registers Affected. CS, PI